This invention relates to methods and apparatus for acoustic well logging in general and in particular to acoustic methods and apparatus for measuring well casing thickness and the quality of the cement bond holding the casing in place.
In the completion of a borehole which has been drilled into the earth in search of petrochemicals, it is common to insert a length or "string" of casing or pipe into the borehole and then "set" the casing by forcing cement into the annulus between the borehole and the casing. This technique is often utilized to separate oil and gas producing zones from each other and also from water bearing strata.
It is known in the art that cement in such an application sometimes fails to provide the desired separation and permits fluids under pressure in one zone to migrate to a second zone. Migration of water into a producing zone is a particularly undesirable occurrence and can possibly render a well unproductive.
Acoustic techniques have long been proposed to determine the quality of the bond between the cement and casing. In one example, Synnott III, U.S. Pat. No. 3,401,773, there is disclosed a technique which utilizes a longitudinally spaced sonic transmitter and receiver. A later portion of the reflection signal generated in Synnott III is integrated and the total amount of energy present in this signal is utilized to provide a measure of the presence or absence of cement. Synnott III exhibits several dificiencies, most notably the inability of the system to provide precise measurement due to the separation of transmitter and receiver and also the system's failure to provide circumferential resolution around the periphery of the casing.
Another technique for evaluating cement bond condition is disclosed in Ingram, U.S. Pat. No. 3,697,937. Ingram discloses the utilization of a sonic transmitter/receiver with zero spacing therebetween in order to enhance the precision of measurements obtained. Ingram utilizes the relative amplitude and phase of reflected sonic energy to evaluate the cement bond.
Further attempts at acoustic investigation of a cased borehole may be seen in the following U.S. Patents: Moran et al, U.S. Pat. No. 3,732,947; Liben, U.S. Pat. No. 3,175,639; Zemanek, U.S. Pat. No. 3,340,953; Norel et at, U.S. Pat. No. 3,883,841; McDonald, U.S. Pat. No. 3,339,665; and, Kaule, U.S. Pat No. 3,741,334.
Most recently, Havira, U.S. Pat. No. 4,255,798 has described a system utilized to measure cement bonding as well as casing thickness. Havira discloses several techniques which employ an acoustic pulse source with a frequency selected to stimulate a radial casing thickness into resonance. The acoustic pulse causes returns or reflections which are formed by reflections from the various interfaces between media of different acoustic impedances. Analysis of these returns by Fast Fourier Transform and integration techniques is then utilized to determine the frequency, as well as energy content of these signals. Casing thickness and cement bond quality may then be determined by an analysis of this data.
Despite the multitude of techniques described in the prior art for acoustic investigation of casing cement bond and casing thickness, those ordinarily skilled in the art will appreciate that a more precise method which permits more accurate measurement of both casing thickness and casing cement bond is greatly desired. The tool utilized in the method of the present invention includes nine sonic transducers which each transmit a five microsecond pulse occupying a band of frequencies from two hundred fifty kilohertz to seven hundred kilohertz. Eight of these transducers are mounted on the tool housing in a helical array, with each transducer offset by forty-five degrees from an adjacent transducer and aimed to transmit a pulse normal to the wall of the casing.
The return signal generated with the described apparatus consists of a reflection from the inside surface of the casing and the reverberation signal from the wall of the casing. The decay rate of this reverberation signal is related to the quality of the bond between the cement on the outside of the casing and the casing itself. It can also be shown that this echo signal contains information which may be utilized to calculate the thickness of the casing. Indeed, the frequency of this reverberation is determined by the casing thickness, utilizing the sound velocity in steel.
A ninth transducer is utilized in a preferred mode of the present invention and transmits a pulse along the axis of the tool to a fixed reflector which is mounted at a known distance. The sound velocity and mud attenuation may then be determined for reference information.
The most obvious known technique for measuring the frequency of a short duration signal is the application of a Fast Fourier Transform or the utilization of a counter circuit, as disclosed in the prior art. However, such techniques do not yield a desired level of accuracy. The method and apparatus of the present invention utilizes an autoregressive adaptive filter which requires significantly less computing time and which yields a more accurate frequency determination, even when more than one signal frequency is present. Also, such a filter may be utilized to determine the damping rate of the reflection signal and therefore may be utilized to generate data which may be used to determine the quality of the casing cement bond.